1. Field of the Invention
The present invention relates to methods and apparatus for efficiently feeding back precoding information in a multiple input multiple output (MIMO) system.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is a popular wireless communication technology to multiplex data in frequency domain.
A multiple antenna communication system, which is often referred to as multiple input multiple output (MIMO) system, is widely used in combination with OFDM technology, in a wireless communication system to improve system performance.
When the transmission channels between the transmitters and the receivers are relatively constant, it is possible to use a closed-loop MIMO scheme to further improve system performance. In a closed-loop MIMO system, the receiver first transmits information regarding the channel condition to the transmitter. The transmitter utilizes this information, together with other considerations such as scheduling priority, data and resource availability, to select a preceding unit. In the transmitter, the data streams to be transmitted are precoded, i.e., pre-multiplied by the preceding matrix, before being passed on to the multiple transmit antennas.
In a contemporary closed-loop MIMO precoding scheme, when a transmitter precodes data before transmitting the data to a receiver, the transmitter informs the receiver of the precoding information such as precoding matrix index (PMI) and transmission rank.
The precoding matrix indication (PMI) and rank feedback on a subband basis can result in significant feedback overhead. For example, and assuming 4-bits per subband for PMI and 2-bits per subband for rank, the total overhead for feedback on five subbands is 30 bits. For larger system bandwidths, the system needs to support a larger number of subbands, thus resulting in even larger feedback overhead. Also, for finer granularity of PMI/rank feedback in frequency, the overhead also increases. Therefore, there is a need to improve the PMI and rank feedback mechanisms that reduces the overhead.